Internal friction measurements were performed in different grades of tungsten carbide-cobalt (WC-Co) cemented carbides from room temperature to 1300 K. The results show that WC-Co exhibits an internal friction spectrum composed mainly of a relaxation peak and a high-temperature exponential background. It is shown that these two components are caused by damping mechanisms that take place in the cobalt phase. These damping mechanisms are interpreted by the movements of dissociated dislocations dragging tungsten atoms that are in solution between the partial dislocations. At low temperatures, the dislocations are widely dissociated, they cannot move, the internal friction is weak, and the material is brittle. At high temperatures, the stacking fault area between the partial dislocations is reduced, the dislocations become mobile, the internal friction peak appears, and the material is more tough. The internal friction peak therefore gives reason for the “brittle-to-tough” transition of the composite material.